The present invention relates to a paper of value (securities) having an embedded security element, preferably an optically variable element in the form of hologram, diffraction or interference structures applied to the paper surface. The invention also relates to a method for producing such a paper of value.
To be protected against imitation by means of color copiers, papers of value or securities are increasingly provided with optically variable security elements, in particular holograms. This protection against forgery is based on the color copier's insufficient ability to reproduce the optical properties of the elements.
Various methods are known for applying the optically variable elements to papers of value. They can usually be divided into three categories, namely, gluing, transfer printing and embossing.
By the gluing method, adhesive labels that are initially prepunched on silicone paper, for example, are transferred to the paper substrate. The adhesive labels have a layer structure composed of at least a contact adhesive layer, a self-supporting film of an optically active layer (for example, with a diffraction grid), and a protective layer located thereabove. The thickness of an adhesive label is typically in the range of 50 micrometers, the main part of the thickness being due to the carrier film.
In transfer printing, also known as "hot stamping," the optically variable element is prefabricated on a transfer band and transferred to the substrate in a subsequent working step. The structure transferred to the paper typically has a thickness in the range of a few micrometers. In the case of holograms, the customary layer structure of the element comprises a heat-sealing layer, a layer of lacquer with an embossing, an aluminized layer and a transparent covering protective layer. This layer structure is initially located on the transfer foil, being affixed to the foil by a release layer (e.g. a wax layer). One transfers the band by placing it with the heat-sealing layer on the substrate and activating the heat-sealing layer by pressing with a heated die, so that the element bonds with the substrate. Simultaneously, the separation layer melts, thereby detaching the hologram from the transfer band. The transfer principle is the most frequently applied method today and is in particular also used customarily for applying holograms to plastic credit cards.
The embossing method is mainly suitable for diffraction elements, such as holograms and optical grids. A layer of hardenable lacquer is applied to a substrate that is preferably provided with an extremely thin and reflective metal surface. A press die is then used to emboss the diffraction relief structure into the layer of lacquer. After the lacquer has hardened, the structure is covered with a protective lacquer. The finished element has a layer structure comprising the successive layers of lacquer with the metal layer and relief structure and the layer of protective lacquer.
Each of the known methods and the resulting products has its own special advantages and disadvantages. For example, adhesive labels are technically easy to produce and can be transferred to the intended substrates without any trouble. An extreme disadvantage of adhesive labels for application to securities, however, is that the entire elements can be detached from the substrate and transferred to forged products. For this reason, transfer and embossed elements are preferred for securities applications.
Transfer and embossed elements largely meet the requirements in terms of protection from forgery for securities, but these elements involve a number of production engineering problems in connection with papers of value.
It must be taken into consideration that papers of value customarily have a high-security printed pattern; these patterns are applied in most cases by steel intaglio printing. Steel intaglio printing and related methods require a relatively high surface roughness of the substrate for the inks to bond well with the substrate. However, rough surfaces are extremely unsuitable for the application of optically variable elements, which have little stability. The quality of sensitive hologram structures is affected very adversely by rough surface structures.
It must be heeded that the paper of value is subjected to a very high pressure load in its whole surface during steel intaglio printing. This customarily reduces the optical effect of any optical elements applied prior to printing; the elements can even be damaged or fully destroyed by the proper roughness pressed through from the paper base.
When producing papers of value having optically variable elements, one therefore first provides the paper of value with the printed pattern and then applies the hologram in one of the following method steps, or one divides the application of the elements into single steps, performing the measures not endangered by steel intaglio printing before the printing and the others only after it. One thereby accepted the disadvantages up to now that this direct coupling with the printing process made it impossible to prefabricate unprinted papers of value with optically variable elements in a job-neutral way (stockpile production), on the one hand, and that the application of the optically variable elements requires suitable machines (transfer machines, etc.) for each printing line, on the other hand. The special machines required for each printing line not only increase the cost and the space requirements of the machinery, but also cause a bottleneck at the end of each printing line due to their different production capacity, which can only be compensated by additional machinery.
EP-A 0 338 378 discloses such a system for producing paper products that have both a printed pattern and an optical diffraction element. In a continuous process the paper is first printed in known printing units. Then, as in the described embossing method, a radiation hardenable lacquer is applied and provided with a diffraction structure in one operation. In subsequent operations the diffraction structure is vacuum coated with a reflective metal layer and provided with a protective lacquer.
In other known systems, the operation of applying the hologram is divided into two steps. Following papermaking, the lacquer is applied to the paper surface in a first step. After the paper is printed, the optical grid is embossed in the next step.
U.S. Pat. No. 4,420,515 describes a variant of this bipartite method. A metal layer with an adhesive layer thereabove is first applied to a plastic transfer band having a prepared surface. These two layers form the substructure of the future security element. In the first step the two layers are laminated onto the substrate, whereby the substructure of the element takes on the surface quality of the transfer band under the action of heat and pressure in the laminating operation. In the second step, a printed pattern and an optically acting relief structure are applied to the substrate.
The forced order of printing and applying the optically effective layers or optically effective structures leads, as already mentioned, to a number of serious disadvantages.
A further disadvantage of the known methods is the difficulty of integrating them into the organizational sequence of security printing plants. For security reasons it is virtually indispensable in paper of value manufacture for the printing process, in particular the printing of the serial number, to be the last processing operation before delivery of the papers of value. In security printing plants it is therefore an established custom to prefabricate paper with the corresponding security features, such as watermarks, safeguarding thread and any optical elements, and then to print it. This manufacturing sequence is likewise not possible with the known methods.